Freedom from long-term anticoagulation and a low incidence of thrombo-embolism have provided the porcine valve heterograft with distinct advantages over mechanical valve prostheses. Recent reports, however, have demonstrated unfavorable hemodynamic function, particularly among the smaller prosthetic sizes. To assess this problem we studied 63 patients following aortic valve replacement with the Hancock porcine heterograft by cardiac catheterization both at rest and conditions of elevated cardiac output by isoproterenol infusion.